This invention relates to a method and apparatus for controlling communications traffic and in particular to controlling a source of communications traffic that is coupled to a communications link. The invention is particularly suited to, but not limited to, packet-based communications systems such as broadband Asynchronous Transfer Mode (ATM) systems which carry variable bit rate traffic.
Communications systems typically have some form of congestion management function to allocate the resources, such as bandwidth on a communications link, to users in a way that prevents the system from becoming congested. Typically some form of connection admission control (CAC) responds to new call requests and admits or refuses calls.
Integrated communications systems are now being developed which can carry a variety of types of traffic, such as voice, data and video. Traffic can be a mix of constant bit rate and variable bit rate traffic. Variable bit rate (VBR) traffic poses particular problems for traffic management as the bit rate of the traffic varies during a call. The effect of a number of VBR sources all operating together can result in a wide range of bit rates for the offered traffic. Clearly, if the offered traffic exceeds the available capacity, it cannot all be sent at the same time. Packet-based communications systems often use a cell-shaping function to xe2x80x9csmoothxe2x80x9d the peaks in the traffic to fit the traffic to the available resource, by buffering the traffic until it can be sent. However, delay-sensitive traffic such as voice cannot tolerate buffering and therefore cannot be treated in this way.
Some communications systems transport voice traffic using silence suppression, which suppresses silent periods in a conversation, thereby allowing the bandwidth which would have been occupied by data bits carrying those silent portions to be used by other traffic. The use of silence suppression provides some additional capacity on a communications link but also has the effect of turning the voice traffic into variable bit rate traffic. This makes it difficult to predict the actual sustained cell rate (average bandwidth) for a voice trunk using a fixed coding scheme because the ratio of talk to silence is not deterministic.
One solution is to engineer the communications system such that it reserves sufficient resources to ensure that the voice traffic is properly carried but this is undesirable in that it uses resources that could have been used by other traffic, thereby reducing the efficiency of the system.
The present invention seeks to provide a method and apparatus for controlling communications traffic.
According to a first aspect of the present invention there is provided a method of controlling offered communications traffic from a traffic source, which source emits packet-based traffic carrying a plurality of variable bit rate calls, the offered traffic being for transmission over a communications link, the method comprising:
monitoring current traffic output from the source;
comparing the monitored current traffic with capacity on the link; and
providing a control signal for varying the operational state of the traffic source according to the comparison.
This method can be used to control congestion caused by the statistical behaviour of traffic, such as speech traffic on which silence-suppression is being employed. Employing a reactive, real time, congestion management system allows the system to be engineered much more aggressively than it would need to be if only first line congestion management, such as connection admission controls (CAC), was implemented. This allows the CAC parameters to be set higher, allowing more calls to be transported, while still maintaining quality.
An efficient traffic management system in the Integrated Access system allows quality of service to be maintained for both the voice and data traffic while also achieving efficient bandwidth usage. Such a system also allows the integrated access traffic to meet its traffic contract with the underlying network. To achieve efficiency in the network it is intended to allow engineering guidelines which assume a reasonable average saving for voice traffic by using silence suppression (say a 50% saving) and to provide an effective congestion management system which deals with periods where the offered load temporarily exceeds the capacity of the link.
The method may perform a traffic policing algorithm, such as the generic cell rate algorithm (GCRA), and use the output of that algorithm, which indicates how near the current traffic is to the limits of capacity on the link or the traffic contract, to determine how to vary the operational state of the source. Alternatively, the method can make measurements of the current traffic and use these to determine how congested the channel is and therefore what change is required to the operational state of the source. The method can make measurements to derive parameters which are good equivalents to parameters that the policing algorithm would derive, such as Peak cell rate (PCR), Sustained cell rate (SCR), Maximum burst size (MBS) and the leaky bucket counter (L) used in the GCRA. Using the GCRA directly, or deriving GCRA-equivalent parameters allows the source to more readily conform to the traffic contract for the link. The capacity on the link can be a traffic contract for the link, which specifies the boundaries for the traffic that will be accepted by the link operator, or it can be provisioned traffic parameters for the link.
The preferred embodiment describes the invention in the context of an ATM-based network, but the invention is not limited to use in such a network, and could also be applied to other types of packet or cell based networks such as Internet Protocol (IP) based networks.
This method can be implemented in hardware, software, or a combination of these. Accordingly, further aspects of the invention provide software stored on a machine readable medium and apparatus for performing each of the steps of the method.
Preferred features may be combined as appropriate, and may be combined with any of the aspects of the invention as would be apparent to a person skilled in the art.